1. Field of the Invention
The present invention relates to a bonding method whereby objects to be bonded can be fixed (provisionally adhered) in a short period of time without application of any special treatment such as heating, pressing or ultraviolet irradiation, and yet adequate bonding strength is obtainable upon expiration of a certain period of time, and to an adhesive useful for the method.
2. Discussion of Background
Bonding by means of adhesives is commonly employed in various industrial fields and at home. In recent years, requirements for bonding have expanded, and they include not only the bonding properties, but also various other requirements such as non-toxicity, non-flammability, efficiency and simplicity of the bonding operation, reduction of the time required for pressing after joining the objects and bonding properties not restricted to ordinary objects but applicable also to porous or rough surfaced objects. However, no bonding method has been known which satisfies such various requirements at the same time.
For example, adhesives for industrial purposes include an epoxy resin adhesive, a hot melt adhesive, a rubber-solvent type adhesive, a synthetic resin aqueous emulsion adhesive, a two-part type acrylic adhesive, an .alpha.-cyanoacrylate adhesive, a two-part type urethane adhesive and an ultraviolet curing adhesive. They are used as excellent adhesives for bonding objects in various fields, for example, in the fields of paper packagings, electrical parts, precision machinery, building and construction, and household utensils. However, these adhesives have various drawbacks as follows.
Namely, the epoxy resin adhesive is capable of firmly bonding a wide range of objects, and has been used widely as an adhesive for structures or sub-structures. However, it usually requires a heat or a long period of time for curing to obtain adequate strength. It is possible to complete the curing in a short period of time by properly selecting a curing agent. In such a case, however, there will be a drawback that the useful time will be extremely short.
The hot melt adhesive is heated and melted at a high temperature in its use, and thus requires a heat coating apparatus. Further, the adhesive force after the bonding sharply drops when the temperature approaches the softening temperature of the adhesive. Besides, the type of objects to be bonded by this adhesive is rather limited.
The rubber-solvent type adhesive has drawbacks such that after the application of the adhesive, it is necessary to evaporate and remove a substantial amount of the solvent before bonding, and the waiting time is long and it takes a long time for the bonding operation. Besides, there is a danger of catching a fire, toxicity or odor due to the evaporation of the solvent.
The synthetic resin aqueous emulsion adhesive has various advantages such that, as compared with an organic solvent type adhesive, it creates less air pollution, and it is superior in the safety and hygiene and easy to handle, and that it is suitable for conservation of resources. Therefore, it is utilized in a wide range of fields, particularly for the bonding of porous objects such as wood, paper or cloth. However, the aqueous emulsion adhesive has a fatal drawback that the development of the initial adhesive force is extremely slow because of the poor drying property due to the fact that the medium is water.
The two-part type acrylic adhesive cures in a few minutes to exhibit excellent adhesive properties. However, it has a strong odor, and its useful life after mixing is extremely short, and it is not suitable for the bonding of a wide range of objects.
The .alpha.-cyanoacrylate adhesive can be cured and bonded at room temperature in a short period of time without requiring heating, mixing or ultraviolet irradiation. However, this adhesive is inferior in the impact adhesive strength and in the peel adhesive strength, and thus is poor in the dependability for the bonding performance. Further, it is inferior in the adhesion to a porous object or to an object having a roughened surface such as wood, cloth or concrete material.
It has been proposed to overcome the above drawbacks by adding various modifiers such as a thickener, a plasticizer or a curing accelerator to the .alpha.-cyanoacrylate adhesive. However, fully satisfactory results have not been obtained.
Like the above-mentioned .alpha.-cyanoacrylate adhesive, a 1,1-di-substituted diene compound adhesive has an excellent property that it is capable of being cured in a short period of time. However, it has the same drawback as the .alpha.-cyanoacrylate adhesive in that it is inferior in the adhesion to a porous object or to a roughened surface.
The two-part urethane adhesive is likely to undergo foaming in the presence moisture or water, whereby the adhesive strength tends to deteriorate, thus leading to defective bonding. Further, it has an additional drawback that it is inferior in the water resistance and in the heat resistance even after curing.
The ultraviolet curing adhesive requires an ultraviolet radiation apparatus, and is applicable only for the bonding of transparent materials.